Competitive adsorption between B-casein and B-lactoglobulin (B-Lg) during spray-drying was studied by the new surface sensitive technique using fluorescence quenching of pyrene labelled protein at the powder surface. The difference in competitiveness of B-casein when present as monomers and as associated into micellar like structures were studied. Results were compared with the adsorption of single proteins at the powder surface. The adsorption of monomeric B-casein alone gave an apparent surface load of 1 mg/m2 at a protein concentration of 0.3% dry weight and then remained constant with an increasing protein concentration. In the presence of Ca2+, associated B-casein gave a lower affinity adsorption than monomeric B-casein and did not reach a plateau value, instead it continued to increase with an increasing protein concentration. B-Lg showed a low-affinity adsorption during spray-drying compared to monomeric B-casein, although not as low as associated B-casein. Competitive adsorption between monomeric B-casein and B-Lg resulted in a higher apparent surface load of B-casein than B-Lg at both protein concentrations studied (total 0.3 and 3.3% dry weight). However, in an associated form B-casein was less competitive than B-Lg. At a low bulk protein concentration (0.3% dry weight) B-Lg dominated the powder surface, whereas at a higher concentration (3.3% dry weight) there was little difference between the proteins. The results indicate that the competitiveness of a protein during spray-drying is highly influenced by the ability of the protein to attach and rearrange at the droplet's air?water interface during the spray-drying process.